Method of and means for reducing crosstalk in cable systems



K. E. LATIMER Nov. 22, 1932.

METHOD OF AND MEANS FOR REDUCING CROSSTALK IN CABLE SYSTEMS Filed Jan. 15, 1932 FIG.

FIG?

lNl/ENTOR K 5 LA T/MER A TTORNEV Patented Nov. 22, 1932 UNITED STATES PATENT;OFFICE i KENNETH n. LATIMER, or ALDWY H, LONDON, ENGLAND, AssIoNoa To wEsTnnN: ELEGQ TRIC COMPANY, INCORPORATED, or YORK V I a NEW YORK, N. y, A CORIOBAT ION. r nw METHOD OF ANDM ANS FOR REDUCING oRossTALK IN CABLE sYstrninsv Application filed January 13, 1932, Serial No. 586,263, and in Great Britain January 5, 1931. -I

This invention relates to telephone transmission systems and more particularly-long distance telephone cablesusing voice frequency circuits worked on a four-wire basis or carrier frequency circuits, but is also applicable to open wire lines. The invention. has for its primary Object the reduction of crosstalk, but there are also other objects which will become apparent in the course of v the explanation which follows.

It is well known that the near end crosstalk between two circuits in a cable varies in a very irregular manner as a function ot the frequency, the irregularities arising from the fact that dilferent phase shifts are associated with the capacity unbalances occurring at different points of the cable, so that at certain frequencies, groups of these imbalances will add up to give a high crosstalk, whereas at a slightly different fr-equen cy the phase'relations will besuchthatthis condition no longer holds. It is for this reason extremely diflicult to reduce the near end crosstalk of an installed cable by means of networks connected between the circuits at one'or both ends; in fact,'for suchan at tempt to be successful it is necessary to have available a delay network or its equivalent, the phase shift of which is utilized to simulate the phase characteristics associated with 7 a way that crosstalk currents tend mutually to subtract. Naturally, such aprocessis difficult and cumbersome to such an extent that it is almost impracticable. For this reason any attempt to reduce near end crosstalk must necessarily be undertaken upon lengths of cable shortcompared with wave length e. g. loading sections.

Much the same considerations apply also with regard to far end crosstalk if there is a difference of velocity between the various circuits inthe cable, e. g. between side and phantom circuits. Attempts have been made to reduce the far end crosstalk-on installed cables by a process known as poling? which 50 consists of choosing one of the eight possible methods of'splicing a quad so that the -phan-. tom tojside and side to'side crosstalk may be reduced to a minimum. This process is usually carried out at one splice in the cen+ ter of each .repeatersection and is applied to the reduction of phantom to side and side to side far end crosstalk,a reduction inthe Order 01530% beingv thereby Obtained.

This particular method is not a very sci-1 entific one in that the] crosstalks occurring upon either side ofthe poling point can not be selected toneutralize one another" with any degree of precision, partly on account ofdifferences in phase and attenuation and partly because the only known Ineansof carrying out the selection'is by trial anderror, which is not very effective. With existing methods, when differences of attenuation oc- 1 our between side and phantom circuits, either the phantomto side crosstalk hasibeen 'I'B-I duced' without reducing the side to phantom, crosstalk, 'or vice 'versa, unless the tests are conducted oncomparatively' short lengths of cable. No precautions have hithertobeentaken to secure exact equality of velocity among the various circuits. i

' -According to afeature of the invention far end crosstalk between two circuits whose transmission times. are substantially equalis corrected by the addition of condensers, inductances or resistances separated, by distances long compared. with a wave length,

' from the unbalances which they correct, and adjusted to balance the crosstalk for,. either direction ofatran'smission and-with etherv of a pair of circuits considered as the disturb? ing'circuit.v Q p According toanother feature of the present' invention the balancing of far endcrosstalk on lengths of cablelong compared with a wave length in which the above condition of substantially equal velocities is not in the. first place satisfactorily met is made more practicable and eflicientby making the trans-. mission times (and cut-oil frequencies in the case of Pupin loading) substantially equal on all circuits in a groupwhether they be side circuitsor phantom circuits,and whetli-' or or not a diflerence of attenuation will be.

produced by'so doing, so that the far end 100 crosstalk becomes a simple function of frequency and so that a reduction in the crosstalk maybe effected by use of simple devices, such as condensers or mutual inductanees, connectedto the ends or other points of the; circuits; The methods by which this is done are described below.

According to another feature of the invention' means are provided for making the terminal impedance presented bythe equip ment match the impedance of the line to a sufficient extentto enable. the balancing to be carried outby means" of condensers or other" simple; devices connected between the cir} cuitswhose crosstalk is to be reduced, such balancing devices being applied'to lengths of cable long-compared with'a' wavelength 'Accordingftoi another feature vof the in-. vention means are provided to enable the magnitudes of said balancing condensers "to be determined by a series. ofsimple meas-" urements, even in the' c'ase in which the two circuits .Idifl'er .in attenuations. These and other features will become apparent from.

the description below.

According to anothe feature. of the invention, a communication-cable comprising con I invention; and

The invention will be better understood from the following detailedY description thereof when read'in connection with the accompanying drawing in which:

' Figs. 1 and -2 arefdiagrams employed to aid in a'mathematical explanation of the pecific circuit, arrangements for practicingthe invention. 1 J

With reference to Fig. 1, let'ab and colbe. the two circuits required-tobe balanced, a a 'crosstalk due to capacity imbalance. and G and C balancing crosstalks introduced at the'ends' of the circuit. Let B B B 13 be the circuit losses in nepers. o 1

Then assuming-equalvelocities the condition for crosstalk balance from a to dis that:

' Figs. 3 and 4 illustrate diagran'nnatically.

is as Since the above expressions only involve I differencesf of attenuation, each unbalance such as a can therefore evidently be balanced or represented by two condensers such "as 01 and C provided that the phase conditions are suitable and that the difierence between the attenuations of the circuits does not vary With frequency, so that by suitably choosing the values of the balancing condensers a simultaneous balance of each element of crosstalk is practicable apart from velocity differences. I We can therefore of a repeater section beconsidered as replaced by' two'crosstalks a and a inFig. 2, a need not 'have-thejsame sign as a 5 Let all four possible readings be taken of the capacity crosstalksnecessary to balance the system for the directionoftransmission from left toright, i. e." 1 V (l) Balancing crosstalk c to balance crosstalk from awto d.

(2) Balancing crosstalk C between a and (4) Balancing crosstalk vC between I) and d to balance crosstalk fr'om 0 to b.

The conditions of balance will be not; 1'

Permanent balancing condensers equal and opposite to a and a are required. 7

It is thus necessary to determine only three f let the complek crosstalk C between a it maybe in practice desirable to determine C1, C C and C in order to facilitate testing or calculations or to provide a check.

It will be noted that the readings in the opposite direction of transmission will be du' plications of the readings already considcred so that the balancing of the cable in one direction will hold good in the opposite direction. Naturally readings of crosstalk balanced at the high level end will correspond in the other direction of transmission with readings of crosstalk balanced at the low level end, with the other circuit considered as the disturbing circuit. Thus a phantom to side reading in the east to west direction balanced at the east end will correspond with the side to phantom reading in the west to east direction balanced at the east end. Considering a carrier system in which acircuit is used for transmission in both directions, the frequencies used for one direction being different from those used in the other, it would therefore be possible to balance out the far end crosstalk in both directions simultaneously.

So far, 'no attention has been given to the phases of the individual elements of crosstalk.

'The phase of the crosstalk current due to a small capacity unbalance between the two circuits is, at the point at which it occurs, exactly 90 out of phasewith the voltage at that point, while themagnitude of the current, on circuits of constant impedance, will be proportional to the frequency. Similarly, the voltage induced in the disturbed circuit by a mutual inductance from the disturbing circuit will be exactly 90 out of phase with the current in the disturbing circuit, the magnitude again being proportional to the frequency on circuits of constant impedance. It will be seen from these relations that cross talk due to capacity unbalance and mutual inductance will be slightly out of phase with one another by an amount equalto twice the angle of the line impedance. Usually, on most types of circuits encountered in practice, this angle is sufficiently small to be neglected for the'present purposes, and consequently it may be said that both these types of crosstalk may be eliminated either by inserting capacity unbalance or mutual inductance at the ends of the circuits in the amounts indicated by the above equations. Under certain circumstances however it may be desirable to allow for the phase difference. This may be done in several ways. One way would be to useboth mutual inductance and capacity unbalance in balancing, while another and preferred method would be to use two condensers for each balancing operation in conjunction with a phase splitting device which could be made to serve several balancing condensers. Thus in Fig. 4, 1, 2 and 3 are three cable circuits, &6 are three-phase splitting devices, 7-9 are terminal equipments and 10-13 are balancing condensers, phase split ter i rotates the phase of the currents passing in condensers 11 and 13 by the required amount. Many similar methods are possible.

Resistance unbalance crosstalk on the other hand, leads to crosstalk voltages which are in phase with the inducing current. This type of crosstalk only occurs in the case of phantom to side crosstalk and in this case if the resistance unbalance crosstalk-is sufliciently high'it may be necessary or desirable to correct it by inserting a small resistance in series With one wire of the side circuit" in question.

The importance of making the terminal impedance of the circuit exactly equal to that of the line may be seen from the fact that if a.

reflected current in the disturbing-circuit passes the balancing device it'also will cause a crosstalk in the disturbed circuit. This effect will be particularly troublesome if the impedance of the equipment differs from that was employed in v of'the crosstalk set which balancing the cable.

The degree to which the impedance of the terminatingequipment should match that of the line depends on the magnitude of the reduction in far end crosstalk required. .Thus if it is required to reduce crosstalk to onetenth of its original'value, thenthe impedance of the terminal equipment must in general match that of the line within a singing-point of 206%, while'if it is required to reduce it to one-thirtieth a singing point of 30d?) is re-' quired, andso on. This is'a considerably higher standard than has hitherto been considered necessary on four-wire voice fre -I matching between the line and equipment I may however be utilized as'a phase splitting device. If it is desired to obtain this effect,

the circuit of Fig. 4 could be used, the devices l 4, 5 and 6 being in this case attenuating pads having the same impedance. as the cable.

The equalization of velocities necessary for practicing the invention can be obtained by several different methods The equalization of velocities of circuits of .a given type, e. g. side circuits, maybe carried outby crosssplicing the cable at one or more points within the repeater section in accordance with total capacity and total inductance measurements, or alternatively, velocity .measurements which may conveniently be made by N the -open circuit, short-circuit impedance method. The splices would be. carriedout in such a way that upon all circuits of a given type the product of total capacity and total inductance would be as nearly as possible the same. i

To control the relative velocity of side and phantom circuits, other devices may be necessary. One method is to vary the average inductance of the phantom coils to give the same average velocity on the phantom circuits as is expected'tobe obtained on the side circuits. As the phantom to side capacity ratio may vary from 1.58 to 1.7 according to the methods of manufactureof the cable, 'gaugeof conductor and even the cable fac' tory,'it will be necessary in certain cases to adopt'slightly different inductances for the phantom circuit, according to circumstances. Care 1s ofcourse normally exercised to msure that the phantom to side capacity ratio remains constant in successive loading sections. Should the average phantom to slde ratio depart-from the expected value, it may be controlled in the field by'the insertion of small amounts ofcable having a difl erent lay-up. For example, ifthe main cable is of quadded. (i. e. multiple twin) type, a small quantity of spiral-4; cable maybe included at regular intervals, for instance in every loading section,'either in such a way as to convey the speech currents or connected to the main cable means of a T splice so as to increase the capacityof the section. As the phantom to side capacity ratio of spiral-4 cableismuch higher than that of multiple twin cable, it is evident that the loading section phantom to side capacity ratiocan be conveniently controlledby this procedure. Another version of this arrangement is to use a much larger amount of spiral-4' cableso that the resulting loading section phantom to side ratio is exactly 2 1 as by this procedure the phantom circuits can be made to have exactly the same velocity and attenuationas the side circuits, so I simplifying the balancing of phantom to pair and pair to phantom crosstalk." As almost equal quantities of multiple twin and spiral-4 cable are required for this arrangement, it would be possible to use a cable in which half'the quads were of multiple twin construction and half of spiral 4 construction, it being arranged that' in any loading section a given circuit would consist of wires laid up according to each construction for exactly half the section. 1

, This'- scheme has other advantages apart from the questions of balancing, as side and phantom circuits would be capable of use for the same distances so that. maximum performance would be obtained for a given cost of cable. I

The actual arrangement of the balancing devicesat the ends of the circuit will now be given circuits by connecting one terminal to a horizontal wire and one terminal to a' vertical wire, the condenserbeing placed in a compartment conveniently situated with re gard to the intersection ofthe horizontaland vertical wires. I

It is not essential that allcircuit combina tions should be balanced or that' balancing should take'place immediately after the in-ov stallationof the cable. It will, for example, often be convenient to install a cable without special end balancing, merely taking the necessary precautions during installation to insure that all velocities are equal.) Subse 7 quently, when a higher standard of perform ance is required from the cable, the balancing condenser can be added, either from every circuit to every other circuit or, if desired, from each important circuit .to every circuit that has appreciable crosstalk to this circuit.

In the case of a quaddedcable, a similar device would be .used to house the balancing condensers, but it wouldthen be desirable to adopt some special procedure to enable different types of crosstalk to be balanced-without interfering with other types of crosstalk. Thus, each condenser may be considered to be replaced by a network of condensers designed to neutralize the unbalance in question without affecting other unbalances. 'An

so i A alternative procedure to this would be to em- 7 ploy simple balancing condensers in conjunction with the normal phantom repeatingcoil groups. When it is required to balance a crosstalk from a phantom circuit to another circuit, a connection to phantom circuit would be obtained by joining the condenser to the center tap of the side-circuit repeating coil.. Provided that all pair or pair bal-' ancing is carried out before the measurement and balancing of phantom to phantom crosstalk, no interference will be obtained between.

the functions of the various condensers.

It is known that crosstalk or babble conditions in a cable circuit are greatest if the circuit be situated near the center of the cable.

For this reason it is customary to build a cable, comprising both two wire and four wire c1rcu1ts,.w1th the conductors for the four wire circuits, which are the high quality circuits, occupying the outside JlayerS.

(Often the very highest qualitycircuitsare provided in the center of the cable, but in such case a special screen protects each of these circuits from all other circuits). By adopting the balancing methods set out above such a reduction in babble or crosstalk can be effected on the four-wire circuits that it may be advantageous to adopt a different allocation of the circuits in the cable. According to a feature of the invention therefore, the.

two-wire circuits of a cable are conductors situated in the outer layers surrounding the conductors of the four-wire circuits, the babble or crosstalk in the latter being balanced by the above described methods. At least three effects are obtained by this rearrangement. In the first place the fourwire circuits of high quality, which unfortunately happen to be the circuits most sensitive to noise, are situated in a region of less noise and interference from outside sources. Secondly, because there are less capacity changes due to cable laying in the outer layers of a cable, the singing point of thetwowire circuits is improved. Thirdly, if

. screening between conductors of the fourwire circuits transmitting in opposite directions is to be provided, this may be furnished 7 more economically, as the screen need only conditions and means or methods for reducing them. It may, however, be possible that other types of disturbance may be reduced by similar or analogous expedients, for example, provided that the velocity of transmission of noise voltages to earth is made the same as that of metallic noise voltages, reductions of noise may be effected by similar means.

What is claimed is: V

1. In a transmission system, a communication cable comprising conductors forming two-wire and four-wire circuits, said fourwire circuits being nearer the center of the cable than the two-wire circuits, and means for reducing far end crosstalk between said circuits comprising balancing impedance elements connected between certain of said circuits at the near end of said cable. 7 v

2. The method of reducing far end crosstalk between the signaling circuits of acommunication cable employing repeaters at the ends thereof, which consists in first equalizing the transmission times of the signaling circuits of a given type by cross-splicing the cable at one or more points with in a repeater section to make the total inductance and capacity of said given type circuits substantially the same, and then balancing the far end crosstalk with reactive elements connectedbetween certain'of said signaling circuits at the near end of said cable.

3. The method of reducin far end crosstalk between the side and p antom circuits of a quadded communication cable, which consists in first equalizing the transmission times of the side and phantom circuits by suitably adjusting the average inductance of crosstalk between said circuits by suitable in ductances and capacities connected therebetween at the near end of said cable.

4. In a signal transmission system, a quadded communication cable in which substantially half the quads are of multiple twin construction and half of spiral-4; construction so that each signaling circuit comprises alternate lengths of the two constructions, and

means for reducing far end crosstalk between the signaling circuits comprising balancing the phantom coils, and then balancing the reactive elements of suitable value connected therebetween at the near end of said cable.

In witness whereof, I hereunto subscribe my name this 3rd day of December 1931.

' KENNETH E. LATIMER. 

